When bronzed sun worshippers bask on the beach they may get more than a tan. Ultraviolet light causes mice to churn out an opiate-like molecule, scientists report in the June 19 Cell. This feel-good molecule, called beta-endorphin, may explain why some people seem addicted to tanning.

The results may also explain why people are more generally drawn to sunny spots, says dermatologist and public health scientist Steven Feldman of Wake Forest University School of Medicine in Winston-Salem, N.C.

“Do you know why people go to the beach on vacation? Why they put Disney World in Florida and not in Minnesota, where it’s cooler? Why caves are not more popular as a tourist destination? It’s all because of what these guys studied,” Feldman says.

The project began with a puzzle, says study coauthor David Fisher, an oncologist at Massachusetts General Hospital and Harvard Medical School. He and his colleagues wanted to understand why skin cancer rates were increasing, even though the culprit, UV radiation from the sun and tanning booths, had been known for years. “We know it’s dangerous,” he says, “and yet we seem to be failing” to protect ourselves.

Fisher and his colleagues exposed mice to a moderate amount of UV radiation for five days a week for six weeks. The exposure was comparable to what a fair-skinned person would receive from 20 to 30 minutes of summer sun in Florida. (The UV light reached the mice’s skin because the researchers had shaved the animals’ backs.)

After about a week, the UV-exposed mice had higher levels of a molecule called beta-endorphin, which acts similarly to morphine and heroin in the body. Seven days after the mice stopped the UV treatment, beta-endorphin levels fell to baseline. The endorphin boost was found in the mice’s blood, not just in skin as earlier work had suggested. Endorphins circulating in the blood are able to carry feel-good messages to the brain.

As beta-endorphin levels rose, so did the mice’s ability to withstand pain. After UV treatment, mice could tolerate a stronger poke to the paw and kept their paw on a hot plate longer. Mice bred so that they could not produce beta-endorphins did not respond to UV light in the same way.

Blocking the effects of beta-endorphin with naloxone, the same drug used to treat heroin overdoses, also blocked the boost in the mice’s pain threshold, the researchers found. That suggests that these UV effects depend on endorphins.

What’s more, mice seemed to get hooked on the high levels of beta-endorphins. After getting used to regular doses of UV light, mice showed signs of withdrawal when their endorphins were silenced with naloxone: The animals’ paws trembled, their teeth chattered and they shook like wet dogs. UV-treated mice also avoided a room in which they had previously received naloxone.

Mice that weren’t exposed to UV light didn’t seem to go through naloxone-related withdrawal and didn’t avoid the room where naloxone was administered.

The results in mice mimic those from experiments on people. In a study conducted by Feldman and his colleagues, frequent tanners chose between two tanning beds that looked identical, one that delivered UV light and one that filtered it out. After trying out both beds, subjects almost always picked the active tanning bed.

As in the mice, this UV-light-seeking behavior seemed to hinge on endorphins: When people’s endorphins were blocked with a different drug called naltrexone, they could no longer distinguish between the real and fake tanning beds, a failure that suggests they had become immune to the rewards of UV light. And in some frequent tanners, naloxone caused signs of withdrawal.

Other research, by addiction psychiatrist Bryon Adinoff of VA North Texas Health Care System and the University of Texas Southwestern Medical Center in Dallas and colleagues, found that UV light boosts activity in the brain’s striatum, a region associated with addiction.

The results on mice are “fascinating” but don’t recapitulate the complexities of human addiction, Adinoff says. “Physiological dependence is not addiction,” he says. “Addiction means craving, loss of control and tolerance, and they didn’t look at any of those things.” Still, he says, tanning can become addictive for people, and the same beta-endorphin production described in mice may be part of what compels some people to tan. “There are a lot of people out there who tan, either on the beach or in indoor salon tanners, and many of these folks report symptoms that are consistent with addiction,” he says.

A clear biological explanation of why some people crave sun may eventually prompt policy makers to subject tanning salons to tougher regulations, Fisher says. Membership policies that offer discounts on multiple sessions, for instance, could be discouraged. “They say, ‘We’ll give you your first ten treatments for pennies,’ and then you’re hooked. You’re literally hooked,” he says. “It’s a great business model but a devastating public health one.”

Editor's Note: This article was updated July 8, 2014, to correct the drug used to block endorphins in experiments involving people.